1. Field of the Invention
This invention relates to a method of reducing the content of reactive gaseous impurities, such as SO.sub.2, NO.sub.x and other reactive gases, from a flue gas or other fluid stream by utilization of a thermally crushed alkali metal or alkaline earth metal carbonate compound (in a variety of forms), the small particle size of the thermally crushed material improving its ability to remove the reactive gas. More particularly, this invention relates to the removal of reactive gases from a flue gas, particularly SO.sub.2, by introducing the thermally crushed small particles or powder into the flue gas when the flue gas is a temperature up to 1,500.degree. F.
2. Description of the Prior Art
Sulfur dioxide is one of the major pollutants from the burning of sulfur bearing fuels. The major tonnage of sulfur dioxide is discharged to the air from steam power plants and furnaces which typically burn coal and residual oils ranging in sulfur content from one to four percent and produce a flue gas concentraion of sulfur dioxide ranging from 800 to about 3,000 parts per million. Metal ore smelting furnaces also generate SO.sub.2 with concentrations up to 20,000 ppm. It has been estimated that some 25 million tons of sulfur dioxide per year are poured into the atmosphere of the United States. This has caused a tremendous pollution problem and an onslaught of proposed techniques for reducing the sulfur dioxide content of flue gases or stack gases from burning combustible products.
In attempts to remove the sulfur dioxide from the gaseous streams, a number of different processes have been proposed and tried. There are numerous suggestions that the sulfer dioxide can be removed from gases by washing the gases with aqueous solutions of alkaline materials. Unfortunately, field trials using these wet scrubbers have been somewhat disappointing and the sulfur dioxide removal achieved has been disappointing. Due to the failure of such wet scrubbing systems, considerable attention has been directed recently to solid systems or other types of systems where the sulfur dioxide can be removed from the gas without using an aqueous scrubbing medium.
For example, it has been suggested that sulfur dioxide can be separated from a gas on solid absorbants such as calcium and sodium carbonate. Because sodium carbonate reacts slowly with sulfur dioxide at normal flue gas temperatures, even more recent proposals have suggested that the flue gas can be contacted with a molten bath of sodium carbonate, with the higher temperature enhancing the rate of reaction.
Still further, very recent attention has been given to the injection of fine powders into a furnace so as to react with or absorb the sulfur dioxide, removing it from the flue gas. However, reports of a maximum of only about 40 percent sulfur dioxide removal through this procedure have been made. One of the principal materials proposed for such injection is limestone powder.
The removal of sulfur dioxide and particulate matter from flue gases through the use of an alkali metal bicarbonate has been suggested in U.S. Pat. Nos. 3,505,008 and 3,589,863 to Ludo K. Frevel and Leonard J. Kressley. The former patent suggests the removal of fly ash and sulfur dioxide from a gas stream by flowing the gas stream through a thin porous bed containing alkali metal bicarbonate crystalline solids having a mesh size predominantly within the range of from about 20 up to about 120 mesh. The latter patent suggests the removal of sulfur dioxide and particulate matter from a gaseous stream by contacting the gaseous stream with porous alkali metal bicarbonate aggregates. While each of these methods is somewaht effective in the removal of sulfur dioxide from a flue gas, neither method is sufficiently effectve to be commercially attractive. Accordingly, the art has still sought a method for the removal of sulfur dioxide from a flue gas which can effectively eliminate the sulfur dioxide impurities in great amounts. This has been achieved through the method of the present invention.
In a similar manner, various processes have been proposed for the reduction of NO.sub.x (NO and NO.sub.2) content and for the reduction of other reactive gas pollutants. NO.sub.x is a chief pollutant in the stack gas of a nitric acid plant, while SO.sub.2 and NO.sub.x are together chief pollutants in power plant stacks. Until the present invention, the art still sought a sucessful method of reducing these pollutants. This has again been achieved in accordance with the present invention.